I think it might help but do you think it would be enough?
This one goes to the cigagette lighter and I notice even the BMW approved one does to .
If you could get one that doubles as a window shade that would be better.

I wouldn't use the lighter socket as most of them are switch power, at least they are on mine X5 but not sure about mine 328i. Better to connect at the back of the battery like the one you referred to (but only ~400mA). You can parallel panels to increase your charge rate but the panels need circuitry to protect from overcharging the battery.